A series of phenylpyridine (ppy)-based 6/5/5 N*C<^>N<^>O and biphenyl (bp)-based 6/5/6 N*C<^>C*N Pt(II) complexes employing tetradentate ligands with nitrogen or oxygen atoms as bridging groups have been developed. Ligand structural modifications have great influences on the electrochemical, photophysical, and excited-state properties, as well as photostabilities of the Pt(II) complexes, which were systematically studied by experimental and theoretical investigations. The timedependent density functional theory calculations and natural transition orbital analyses reveal that Pt(bp-6), Pt(bp-7), and Pt(bp-8) have dominant ligand-centered ((LC)-L-3) mixed with small metal-to-ligand charge-transfer ((MLCT)-M-3) characters in T-1 states, resulting in relatively low quantum efficiencies (Phi(PL)) of 5-33% and 12-32% in dichloromethane solution and PMMA film, respectively. By contrast, Pt(ppy-1) possesses much more (MLCT)-M-3 character in the T-1 state, enabling a high Phi(PL) of 95% in dichloromethane and 90% in DPEPO film, and large radiative decay rates. The strength of the Pt-N-1 coordination bond plays a critical role in the photostability. Pt(ppy-1)- and Pt(bp-6)-doped polystyrene films demonstrate long photostability lifetimes of 150 min for LT97 and LT98.5, respectively. A Pt(ppy-1)-based green OLED using 26mCPy as host realized a peak EQE of 18.5%, which still maintained an EQE of 10.4% at 1000 cd/m(2), and an L-max of over 40 000 cd/m(2) was achieved. This study should provide a valuable reference for the further development of efficient and stable phosphorescent Pt(II) complexes.